The present invention relates to a pressure gauge, and more particularly to a pressure gauge adapted for use in measuring the pressure of a thixotropic fluid subject to aggregation stagnation, such as a liquid (magnetic liquid) in which magnetic material is in solution.
Many types of pressure gauges are known. In one of the known pressure gauges, a diaphragm is provided at the pressure introducing portion of the gauge. The amount of deformation of the diaphragm when it is elastically deformed by pressure is used for determining the pressure of the gas or liquid to be measured.
An example of a diaphragm-type pressure gauge is illustrated in FIG. 4. In this pressure gauge, a T-pipe 11 having an inlet A, an outlet B, and a conduit 12 for introducing liquid to a diaphragm acting as a pressure sensor 13 of a pressure indicator 2 is inserted into a pressure gauging portion of a pipe 14. The pressure indicator 2 is attached to the conduit 12. Assuming that the fluid flows through the pipe 14 from the left side to the right side in the figure, the fluid first flows from the inlet A into the T-pipe 11, and is divided into two routes, one destined for the conduit 12 with the pressure indicator 2 attached thereto and the other destined for the outlet B.
As a result, the fluid fills the conduit 12 to push the diaphragm of the pressure indicator 2, thereby to indicate a pressure.
The pressure gauging system which introduces the fluid to the pressure sensor 13 requires space for introducing the fluid, i.e., the conduit 12 of the T-pipe in the pressure gauge of FIG. 4. In the vicinity of the portion where the conduit branches from the pipe, there is little flow of the fluid filling the space due to disturbances caused by the flow of the fluid in the pipe. The fluid flow gradually diminishes away from the branch portion. Since here is little flow, stagnation occurs in the vicinity of the diaphragm.
This phenomenon becomes remarkable as the viscosity of the fluid flowing through the pipe increases, thereby more significantly impeding the fluid flow. This phenomenon does not cause much of a problem when the fluid is gas, water, or a solution of low concentration. However, magnetic fluids containing a mixture of a magnetic substance, chemical destaticizer, lubricant, binder and the like have a high viscosity and poor fluidity, and hence have a strong tendency to stagnate in the vicinity of the diaphragm. The magnetic liquid generally has thixotropic properties. When it is flowing, a magnetic liquid retains a sol-like fluidity due to the shearing force applied to the fluid due to the flow. However, when such a solution stagnates, its consistency becomes gel-like, so that aggregation occurs.
As the aggregation progresses, the diaphragm of the pressure sensor 13 fails to perform its function. A pressure measured under this condition is not correct, so that accurate pressure control in the magnetic material coating process is impossible. Further, the aggregated matter retained in the stagnation region tends to mix with the magnetic liquid as an impurity, so that control of the the magnetic liquid concentration is impossible. Moreover, air that has been carried in together with the magnetic liquid is left in the stagnation region. A disturbance causes air held in the stagnation region to rush into the pipe. The air, together with the aggregated matter, is transported to the coating head. The result is unwanted streaks, bubbles, and the like in the final product.
To manufacture another type of product, the magnetic liquid in the pipe must be replaced by another magnetic liquid. In this case, a fresh magnetic liquid is supplied to the pipe after the inside of the pipe is cleansed by feedinq detergent into the pipe. In the cleaning process, the cleaning fluid tends to collect in the stagnation region, so that insufficient cleaning of the inside of the pipe is obtained. In this respect, the cleaning work is inefficient. As described above, in the pressure gauge for gauging a pressure by introducing the fluid into a pressure sensor of the diaphragm, the conduit 12 forms a stagnation region of the fluid. This stagnation region causes the mixing of the aggregation and air into the magnetic fluid. Reduction of the volume of the fluid introducing portion that defines the stagnation region can be done to control the mixing of the aggregation and air into the magnetic fluid.
A pressure gauge based on the reduction of the volume of the stagnation region is disclosed in Japanese Utility Model Laid-Open Publication No. Hei. 2-9847, for example. In such a pressure gauge, the fluid path is flush with the opening portion where the diaphragm is mounted, and a pressure gauge mounting seat is used, omitting the conduit 12. Further, a restricting portion is provided which restricts the fluid flow gradually from the diaphragm mounting opening portion toward the bottom corresponding to the fluid path. Provision of the restricting portion succeeds in reducing the volume of the space for introducing the fluid present between the diaphragm and the fluid path.
As regards the mounting seat, the space from the fluid path to the diaphragm increases toward the circumferential end of the diaphragm. The fluid flows more slowly in this space than in the vicinity of the fluid path, and hence the fluid is likely to linger in this space. Accordingly, the above pressure gauge also suffers from the above-stated problem. That is, the aggregation of the fluid interrupts the normal operation of the pressure sensor 13 by the diaphragm, impairing the exact gauging of the fluid pressure and precise pressure control in the magnet fluid coating process.
Further, the aggregated matter is mixed into the magnetic liquid as an impurity, making it difficult to control the concentration of the magnetic liquid. Air is also mixed into the liquid. The air, together with the aggregated matter, is transported to the coating head, causing streaks and bubbles in the final product, and hence deterioration of the product quality. In cleaning the inside of the pipe, the cleaning fluid will not flow properly, providing insufficient cleaning. This results in inefficient cleaning work.
In this type of the pressure gauge, a planar diaphragm is mounted in the fluid path (normally tubular) even under the possibly reduced volume in the pressure measuring location, and an empty space essentially exists between the fluid path and the diaphragm. The space provides a stagnation region for the fluid.